Calipers are well known instruments for measuring dimensions of various articles. Vernier calipers employ linear movement adjustable by a fine screw thread with scales to measure up to a ten-thousandth of an inch. Micrometer calipers employ rotational movement by a fine thread to accomplish the same purpose. Calipers are generally constructed in two types, one to measure outside dimensions and the other to measure inside dimensions, e.g., outside or inside diameter of a cylindrical member. In special instances the micrometer is made to measure the outside or inside diameters (root diameter or pitch diameter) of screw threads on a pipe or various large shafts or nuts for ship's propellers, etc. In such instances it is, of course, necessary to have a wide variety of tips on the micrometer caliper so as to fit each of the many types and sizes of screw threads that are used today. The expense of maintaining a plurality of specially machined tips, as used in micrometers of U.S. Pat. No. 1,797,121 to Bohn, U.S. Pat. No. 2,287,097 to Graham, or U.S. Pat. No. 2,249,611 to Johnson, is considerable. Accordingly, it is an object of this invention to provide a much simpler and less expensive micrometer caliper, particularly for internal measurements of large diameter screw threads. Other objects will appear from the more detailed description of this invention which follows.